Dual polarised or X-polarised antennas are today commonly used in cellular systems for mobile communication. The use of such antennas allows the use of polarisation diversity techniques to combat signal fading in the system. Compared to the use of vertical polarised antennas and space diversity techniques the number of antennas needed is reduced to half, which saves costs and reduces the size and the visual appearance of the antenna installations.
One important performance measure for dual polarised antennas is the isolation between the two antenna ports feeding the two polarisations. Typically, an isolation of more than 30 dB between the ports is wanted, which corresponds to a power coupling of less than 1/1000 between the ports.
An aperture coupled patch antenna element is a commonly used antenna type for dual polarised systems. In aperture coupled patch antenna elements, one or more metallic patches are fed by a micro strip feeding arrangement through a cross shaped aperture in a ground plane, as is shown in FIG. 1. Here, the antenna element 101 includes a radiating patch 103, fed through an aperture 109 by a microstrip feed line 105 positioned between a shielding cage 102 and a printed circuit board.
Isolation between a transmitting and a receiving signal path in a dual polarized antenna has been described in, for instance, prior art document U.S. Pat. No. 6,509,883. According to this document, a signal being transmitted from a first antenna element having one polarisation is received by a second antenna element having another polarisation, thereby causing an unwanted signal to be received by the second antenna element. In order to compensate for this, a compensation path is arranged between the transmitting and receiving signal paths, where the compensation path has a length such that the compensation signal travelling through the compensation path and the unwanted signal have equal magnitude and opposite phase when they meet in the receiving signal path.
Prior art solutions like the one described in U.S. Pat. No. 6,509,883, have a disadvantage in that they only compensate for signals having been transmitted from one antenna element and received by another antenna element. Thus, no solution is shown for solving the problem of capacitive coupling related to the feeders themselves.
In U.S. Pat. No. 6,509,883, the compensation path as well as the transmitting and receiving signal paths have to be adapted to have certain lengths in order to be able to cancel out the unwanted signal, having been transmitted from the first antenna and received by the second antenna, since a difference in length of an odd number of half wavelengths has to be present between the paths traveled by the unwanted signal and the compensation signal.
The prior art solution will therefore only cancel out this specific unwanted signal. Other unwanted signals, resulting from couplings other than this one, such as unwanted signals originating from capacitive coupling between the feeders in a point where the feeders are close to each other, will not be cancelled by the solution shown in this document, since the distinctive length requirements of the signal paths result in cancellation of the unwanted signal only if the unwanted signal and the compensation signal have traveled exactly those required lengths.
Also, a capacitive coupling between the feeders may take place at a very unfortunate point, for which a difference in length of an even number of half wavelength results between the paths traveled by the unwanted signal and by the compensation signal in U.S. Pat. No. 6,509,883. The compensation signal would in this case add to the unwanted signal instead of cancelling it.
Further, due to the signal path length requirements, the antenna element shown in this document has to have a certain size to achieve efficient cancellation, which is disadvantageous.
Thus, there is a problem in prior art relating to cancellation of different kinds of couplings being present in a dual polarized antenna element.